Protein kinases play central roles in the signaling pathways that regulate the growth and proliferation of cells, and aberrant kinase activity contributes to the development of many cancers. Recent success in treating particular cancers with targeted protein kinases inhibitors, notably lung cancer and chronic myeloid leukemia, underscores the importance of these proteins in oncogenesis, and highlights the need for additional kinase inhibitors to treat other cancers. The development of new kinase inhibitors is challenging because the high sequence conservation of the kinase ATP-binding site, the major site targeted by these small molecules, makes it difficult to obtain compounds that are selective for particular kinases. The current study aims to address this problem through an entirely new experimental approach that utilizes advances in physical chemistry. In Aim 1, a new spectroscopic technique called vibrational Stark spectroscopy will be used to construct a map of the electrostatics of the ATP-binding site and how it varies across the ~500 members of this protein family. These measurements will be made using kinase inhibitors that possess vibrational probes of electric field, in which the probes report on the electrostatics they experience when bound in the ATP- binding site. Because these electrostatic maps relate to how the physical environment in the ATP- binding site appears from the perspective of the inhibitors, they will yield direct insight into how changes to the chemical structure of the inhibitors would affect the interaction with kinases. Differences uncovered between kinases in these measurements could be exploited to design more selective drugs. In Aim 2, this possibility will be quantified by performing large-scale binding assays in which the selectivity of panels of kinase inhibitors will be revealed and directly compared to the electrostatics measurements to reveal how electrostatic variation dictates selectivity. While selectivity profiling is commonplace in the pharmaceutical industry, the comparison with the electrostatic maps determined in Aim 1 will allow the physical basis of inhibitor selectivity to be determined for the first time, guiding the way to the development of inhibitors with new selectivity profiles. In Aim 3 the characterization of the ATP-binding site will be completed by studying how this environment is affected by the dynamic rearrangements of protein groups and bound water molecules. The protein kinases now constitute a major group of pharmacological targets, and taken together this work will constitute the first comprehensive experimental study of how the physical properties of these proteins dictate their interaction with drug molecules.